Mold System for Casting of Reactive Alloys

ABSTRACT

The present invention relates to novel binder and slurry formulations used to form molds for casting metal alloys and, more particularly, reactive metal alloys. The shell molds lead to more uniform castings which exhibit limited alpha case as compared to other currently available shell mold systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/938,374 filed on Nov. 12, 2007 which claims the benefit of U.S.Provisional Application No. 60/865,226, filed on Nov. 10, 2006. Thedisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to slurry compositions and molds madetherefrom for casting metal alloys, particularly reactive metal alloys.

BACKGROUND OF THE INVENTION

It is known that suitable refractory systems for the investment castingof reactive alloys such as titanium, zirconium, etc., are very difficultto achieve due to the extremely reactive nature of such alloys. Thesealloys will reduce most oxides on contact during the casting processyielding extensive gas defects and causing solubility of oxygen in thesurface layers of the casting thus requiring extensive chemicalmachining to remove this layer as it is brittle and renders the castingsunsuitable for use in most applications. This is a very serious problemin castings being used in aerospace applications for example. Currentyttria systems have been developed and are being used commercially, butthese systems depend on short-lived prime coat slurries, which must becarefully controlled and can only be maintained for one week or less.Thereafter, such slurries tend to severely degrade.

In the yttria prime slurries currently used in the marketplace, theapproach to overcoming the tendency for ions liberated by the relativelyrapid dissolution of yttria in lower pH condition is one of two generalmethods. In one, additions of a large organic bases such astetraethylammonium hydroxide for example to a yttria slurry helpsprevent the dissolution of silica at the high pH levels necessary tokeep the slurry stable. Other approaches have been to “alloy” the yttriawith other non-reactive oxides in a fusion process which tends to reducethe number of dissolution sites on the yttria particles or to coat theyttria with large adsorbed organic molecules to achieve the same result,both methods contributing to the overall stability of the system. Whilethese methods work to some degree, there is still a need for reactivealloy mold systems that require limited amounts, if any, of suchadditives and can utilize commercially available refractory materialssuch as pure fused yttria or sintered yttria material to produce astable slurry with a relatively long shelf life. In the presentdisclosure, such a system is presented.

SUMMARY OF THE INVENTION

The slurries of the present invention and, ultimately, the investmentcasting molds made therefrom comprise refractory material; low silicacontent colloidal sol binder including an organic polymer; and,optionally, one or more of the following: cross-linking agent, sinteringagent, surfactant, anti-foaming agent, and, stabilizing acid. As will bediscussed in greater detail below, the low silica content colloidal solmay be positively charged, negatively charged or neutral depending uponthe application.

By employing the novel slurries of the present invention to form moldsor “shells” as they are otherwise known in the art to cast reactivemetal components, components having improved casting quality, and whichexhibit low alpha case, can be readily formed.

DETAILED DESCRIPTION OF THE INVENTION

While the slurry compositions described herein are generally used asprime coat slurries as the term is understood in the art, thecompositions described herein may also be useful as intermediateslurries under certain circumstances with minor modifications as will beexplained below.

In the preparation of a casting shell used to mold a metal object, atleast one application of the prime slurry is coated onto a wax pattern,the excess slurry is drained from the pattern and then the part istypically stuccoed with any suitable refractory grain, including, butnot limited to fused yttria, fused alumina and fused zirconia.Thereafter, additional layers of prime coats and/or backup coats, withor without stuccoing, as is known in the art, are applied to the greenshell formed from the prime slurry until the shell has the desiredthickness.

As noted above, the slurries of the present invention include refractorymaterials which are suitable for forming shell molds used to castaerospace, medical, marine and sporting good products for example, outof reactive metals such as titanium and zirconium, as well as productsformed from less reactive alloys such as Inconel® 718, 738 and 792; Rene80 and Mar M 302, by way of non-limiting example. As such, therefractory material preferably is one that not only leads to strongshell molds but also one that is capable of limiting oxide reductionupon contact with the molten metals being cast. The examples of usefulrefractory materials include yttria, zirconia and rare earth metaloxides. Preferred refractory materials include fused yttria, sinteredyttria, fused zirconia, fused calcium zirconate, Er₂O₃, Sc₂O₃, Pr₂O₃,SrO, La₂O₃ and mixtures thereof. Preferably, the refractory materialwill have an average mesh size of between about −200 to −325, such meshsizes being known in the art as “flours”. The refractory material istypically present in the slurry in an amount of between about 70.0 to90.0 wt. %.

Depending upon the intended use of the shell, the low silica contentcolloidal sol binder used may be positively charged, negatively chargedor uncharged, e.g., neutral depending on the application. When using apositively charged silica sol, the silica particles will remain stableat virtually all pH levels as the dissolution ions of the refractory,being themselves positively charged, will not attach to the solparticles, causing their charge to neutralize and thus coalesce losingtheir binding ability. Examples of commercially available positivelycharged colloidal silicas useful in forming a sol with the addition ofdistilled water include Bluonic PB 9430 available from Wesbond and WesolP also available from Wesbond. The positive silica sol will generally bepresent in the slurry in an amount of between 0.5 to about 20.0 wt. %and, preferably, between 2.0 and 10.0 wt. %.

An organic polymer is also generally employed. The organic polymer ispreferably a soluble polymer such as polyvinyl alcohol. Highly preferredpolyvinyl alcohols will have a hydrolysis level of between about 85 to100%. A fully hydrolyzed grade of polyvinyl alcohol (PVA) is preferredbecause it is less susceptible to resolution on subsequent wetting ofthe layer during the shell build process. One example of a commerciallyavailable polyvinyl alcohol composition useful in the slurries of thepresent invention is Celvol 08-125 available from Celenese Corp. Theorganic polymer is generally present in an amount of up to about 2.0 wt.% (dry weight basis) of the slurry.

In addition to the foregoing, slurries employing a positive sol willalso generally include a stabilizing acid, preferably glycolic acid orhydroxypropyl cellulose, by way of non-limiting example. The stabilizingacid is generally present in quantities of up to about 2.9 wt. % of theslurry, and preferably, between about 0.1 and 1.0 wt. %.

In addition, a cross-linking agent may be used to limit resolution ofthe slurry after drying. The cross-linking agent, when used, will bepresent in an amount of up to about 1.0 wt. %. Useful cross-linkingagents may be glyoxal dialdehydes, a polyamide-ephichlorohydrin resin orzirconium ammonium carbonate for example. Curesan 200 available fromBASF Corporation, is a useful commercial example.

Surfactants and/or anti-foaming agents may be used in sufficientquantity to wet the wax pattern and eliminate air from the slurry. Asshould be appreciated by those skilled in the art, the surfactant andanti-foaming agents are generally only present on the order of 10 orfewer drops per 100 g of the slurry. Preferably, surfactants arenon-ionic type surfactants and preferred anti-foaming agents aresilicone based.

The liquid portion of the slurry, namely, the silica sol, the organicpolymer, the stabilizing acid (if any), the cross-linking agent andwater may be in the form of a pre-mixed binder composition having anoverall solids content of between about 25 to 32%. Preparation of thispre-mixed liquid binder simplifies the preparation of the slurry,especially under production conditions.

A small amount of a very finely ground refractory sintering aid, such aszirconia or alumina, by way of non-limiting example, may also beemployed. By “very finely ground”, it is meant that the sintering aidwill have an average particle size of less than about 2.0 microns. Thesintering aid is generally present at between about 0.10 and 3.0 wt. %.The refractory sintering aid may be added freely to the slurry orpre-blended with the refractory material such that the dry pre-blendedrefractory mix can be added to the liquid binder ingredients in a singlestep to form the slurry. Use of these pre-blends simplifies the slurrypreparation while allowing additional user control over alpha case, suchas when zirconia, yttria and sintering aid are pre-blended to minimizecosts while still maintaining an acceptably low level of alpha case.

Slurry formulations employing positively charged sols are set forth inExamples 1-6 presented below.

Under an alternate embodiment of the present invention, negativelycharged or neutral silica sols may be employed to form stable slurriesuseful for making reactive metal casting shells. Such slurries have beenfound to remain stable in a pH range of 8 to 9 without significant agingtendencies. Examples of commercially available negatively chargedcolloidal silicas useful in forming a sol with the addition of distilledwater including Bindzil 830 available from Eka Chemical. Further, auseful commercially available neutral silica is Bindzil DP-5110 is alsoavailable from Eka Chemical.

Slurries employing negatively charged or neutral silica sols, include,in addition to a major amount of refractory material as described above,an organic polymer. Here, however, the organic polymer may includelatex, polyvinyl alcohol, hydroxypropyl cellulose and polymer emulsionsbased on styrene, butadiene, acrylics, and vinyls, among other polymersand co-polymers. For the negatively or neutral charged sols/slurries theorganic polymer content may be higher than for the positively chargedsols. In this regard, the organic polymer component may be up to about3.0% on a dry weight basis.

To render the slurry more resistant to re-solution, one of severalcommercially available organic cross-linking agents may be added to thenegatively charged or neutral sol based slurry to react with the polymerand binder. Along with the foregoing slurry additives, surfactants areagain generally required to enhance “wetting” of wax patterns used inthe process, and small amounts of an anti-foaming agent may be used, ifnecessary. It should be noted that while weak organic acid additions aregenerally used to enhance the stability of the positive sol slurries, nosuch acid additives are to be employed with the negatively charged orneutral systems as this could lead to binder gellation or acceleratedaging.

Again, a minor amount of a very fine refractory oxide, e.g. zirconia,etc., may be added to enhance the fired strength of the coating layerand to avoid any possible surface erosion during the casting process.Other such oxides, alumina for example, could also be used. These oxidesreact with the refractory component, particularly yttria inclusiverefractories, during the mold sintering process, which occurs prior tocasting.

Slurry formations employing negatively charged or neutral sols are setforth in Examples 9-14 presented below.

After drying of the prime layer for several hours, a secondary orintermediate slurry is applied. Any suitable high temperature stablerefractory slurry as found in published literature may be used. Thebinder system of the secondary or intermediate slurry may be eitheralcohol based, e.g., ethyl silicate, or water based, such as colloidalsilica or other inorganic binders. If a colloidal silica intermediatesystem is used, additions of one of the cross linking agents describedabove may be used to further prevent any dissolution of the prime coatslurry. In this case, inorganic cross linking agents such as ammoniumzirconium carbonate may also be used to limit softening of the primecoat(s) during the dipping process. Preferred amounts of suchcross-linking or insolubilizing agents is between 0.1 to 0.5 wt. %. Oneor more intermediate layers may be applied as desired and would bestuccoed with suitable stable refractory grain similar to that used onthe primary yttria layer.

Suitable intermediate slurry formulations useful with prime slurriesdescribed herein are set forth in Examples 15 and 16.

Several backup slurry layers are then applied. These layers consist ofany suitable system common to the investment casting industry and areused to mainly build thickness of the mold and contribute to thestrength necessary to withstand the rigors of dewaxing of the mold.These layers are usually stuccoed with larger sized refractory grainthan the primary and intermediate coats. Refractory grains useful tostucco the above described slurry compositions may have a grit ofbetween 20 to 70 for example. Suitable refractory stuccos includeyttria, alumina, zirconia, mullite, aluminosilicate, rare earth metaloxides and mixtures thereof by way of non-limiting example.

After the mold building process is completed, a final drying cycle istypically used to ensure that the full designed strength of the mold isdeveloped prior to removal of the wax pattern. The mold is then dewaxedusing any of the commonly used methods in the industry, preferablyautoclave or flash fire dewaxing. These methods are of common knowledgeand have been in use throughout the industry for many years. The dewaxedmold is then inspected and repaired if necessary. Any cracks which areapparent are covered with a suitable refractory cement, and a seal dipslurry applied to the repaired area. The mold is then fired to atemperature of between about 1600 and 2200° F. and held for 2 to 4 hoursto develop fired strength in the mold to withstand the stresses ofcasting. The firing also assists in fully developing strength in thenovel prime yttria coat through sintering of the silica, sintering aidand yttria flour producing a smooth, non-dusty surface which will reactonly minimally with the reactive metal during and after the castingprocess.

Filling of the mold is done under vacuum conditions, using any of theconventionally used reactive metal casting processes. The mold, asdescribed above, may be preheated at any temperature up to 2200° F. orlower, even at room temperature, depending on the mold and partconfiguration and alloy used. As would be expected, lower mold preheattemperatures will produce less metal/mold reaction and, in the case oftitanium alloy casting, less alpha case, an oxygen rich brittle layer,which must be removed before the casting is put into service.

The casting, when cooled, is then removed from the mold via anyconventional method and cleaned and machined as required to fulfillspecifications of the end user.

The following examples are provided to further describe the presentinvention and are note intended to be limiting.

EXAMPLE 1 Yttria Prime Coat Slurry

Material Weight Percent Colloidal Silica (Bluonic (PB9430+) 7.80Distilled water 4.00 Polyvinyl Alcohol (Celvol 08-125) 12.30 GlycolicAcid 0.50 Insolubilizer/cross linking agent 0.20 (Curesan 200) Zirconia(MEI Grade MS-2) 1.80 Fused Yttria (UCM −325 mesh) 75.60 Surfactant(Buntrock PS 9400) 5 drops/100 g slurry Antifoam (Buntrock AF 9450) 2drops/100 g slurry

The ingredients are added and mixed in the order shown above. Preferablya high shear mixer is used to properly disperse the ingredients toachieve the desired results. After initial preparation the slurry ismaintained by slow stirring until an initial stability in viscosity isobserved. The user may use minor additions of distilled water andglycolic acid to adjust the viscosity to that desired. After finaladjustment, the slurry was found to be stable for several weeks if keptunder enclosed stirring. If maintained in an open container, periodicadditions of distilled water to the slurry to account for evaporationmust be made to keep at the desired viscosity.

EXAMPLE 2 Yttria Prime Coat Slurry

Material Weight Percent Colloidal Silica (Bluonic PB9430+) 7.64Distilled water 13.90 Glycolic Acid 0.25 Zirconia (MEI Grade MS-2) 2.08Fused Yttria (UCM −325 mesh) 76.38 Surfactant (Buntrock PS 9400) 5drops/100 g slurry Antifoam (Buntrock AF 9450) 2 drops/100 g slurry

The ingredients are added and mixed in the order shown. No cross linkingagent is used in this case and this slurry is recommended only for usewith organic, i.e. ethyl silicate intermediate slurry.

EXAMPLE 3 Yttria Prime Coat Slurry

Material Weiqht Percent Colloidal Silica (Wesol P) 7.63 Distilled water3.91 Polyvinyl alcohol (Celvol 08-125) 12.04 Glycolic Acid 0.50Insolubilizer/cross linking agent 0.20 (Bercet 2040) Zirconia (MEI MS-2)1.76 Fused Yttria (UCM −325 mesh) 73.98 Surfactant (PS 9400) 5 drops/100g slurry Antifoam (AF 9450) 2 drops/100 g slurry

The slurry is mixed as in Examples 1 and 2. This slurry utilizes apositive silica sol (Wesbond P) and an insolubilizer, Berset 2040, aglyoxol based material to render the slurry insoluble after drying.

EXAMPLE 4 Pre Mixed Binder

Material Weiqht Percent Colloidal Silica (Bluonic PB9430+ 31.3 Polyvinylalcohol (Celvol 08-125) 49.4 (7.5% water solution) Insolubilizer/crosslinking agent 1.1 (Curesan 200) Glycolic Acid 2.0 Distilled Water 16.1

These ingredients are mixed vigorously until homogeneous and then storedin a covered container at room temperature until use.

EXAMPLE 5 Yttria Prime Coat Slurry (Made with Pre Mixed Binder)

Material Weiqht Percent Pre mixed binder (From Example 4) 22.0 Zirconia(MEI MS-2) 1.7 Fused Yttria (UCM −325) 76.7 Surfactant (Buntrock PS9400) 5 drops/100 g slurry Antifoam (Buntrock AF 9450) 2 drops/100 gslurry

This slurry is similar to others in previous examples, but is made usingthe pre mixed binder of Example 4.

EXAMPLE 6 Zirconia Prime Coat Slurry (Made with Pre Mixed Binder)

Material Weight Percent Pre mixed binder (from Example 4) 20.0 FusedCalcium Stabilized Zirconia 80.0 (UCM Ca Stabilized Zirconia −325)Surfactant (Buntrock PS 9400) 5 drops/100 g slurry Antifoam (Buntrock AF9450) 2 drops/100 g slurry

This slurry is made using the pre mixed binder of Example 4, and isuseful for the preparation of molds for casting smaller titanium shapeswhere severe heat input does not occur, resulting in acceptable alphacase. It is worthy to note that many currently used zirconia moldsystems utilize objectionable ammonia based binders as their odor isundesirable.

Although the yttria prime is generally considered the least reactive andmost preferred prime coat system, particularly for casting titaniumalloys, other prime coat systems are also possible utilizing the samebinder technology as taught in this invention. Example 6 shows a primesystem utilizing a fused calcium (lime) stabilized zirconia flour.Primes utilizing zirconia are sometimes used for casting titanium,particularly when a lower cost system is desired, usually for smaller orless critical parts such as golf clubs. Current state of the artzirconia primes generally utilize either zirconium acetate or ammoniumzirconium carbonate as the base binder. Zirconium acetate binders aredeficient in that these coatings are generally somewhat water soluble,or sufficiently sensitive to water contact such that one needs to employextra steps to render the coating insoluble or perhaps totally eliminateautoclave de-waxing. Autoclave de-waxing can be facilitated by the useof ammonium zirconium carbonate as the base binder, but this isobjectionable for health and safety reasons because of the strongammonia odor and inherent toxicity associated with the use of ammoniumzirconium carbonate. The binder technology as taught in this inventionovercomes both of these deficiencies, as there is virtually no odor ortoxicity associated with the use of the positive sol (Bluonic PB-9430)such as shown in example 4. Numerous other refractory materials such asfused or tabular alumina, mullite, various alumina silicates, zircon,fused silica and various other commercially available refractories arealso compatible with this binder and may be employed in prime or back-upslurries as desired depending upon alloy type being cast and/or economicconsiderations.

EXAMPLE 7 Intermediate Cross Linking Aqueous Slurry

Contains a cross linking, insolubilizing agent which also acts as aninorganic binder to be used with the yttria slurries of this invention.

Material Weight Percent Ammonium zirconium carbonate 18.0 Tabularalumina (−325 mesh) 82.0

This slurry will not cause any softening of the yttria prime slurry asthe ammonium zirconium carbonate (AZC) serves a dual purpose as aninorganic binder and as a cross linking insolublizer for any of theabove Examples 1, 2 and 3.

EXAMPLE 8 Intermediate Insolubilizing Aqueous Slurry

This slurry contains an organic cross-linking, insolubilizing agent toeliminate any tendencies of the yttria slurries of Examples 1, 2 and 3to re-dissolve during the dipping process.

Material Weight Percent Colloidal Silica Sol (Bindzil 830) 18.0 Tabularalumina (−325 mesh) 81.8 Insolubilizer/cross linking agent 0.2 (Curesan200) Surfactant (Buntrock PS 9400) 5 drops/100 g slurry Antifoam(Buntrock AF 9450) 2 drops/100 g slurry

This slurry utilizes a more conventional negatively charged colloidalsilica sol as the primary binder.

EXAMPLE 9 Yttria Prime Coat Slurry (Negative Sol)

Material Weight Percent Colloidal Silica (Bindzil 830) 7.28 Distilledwater 11.11 Polyvinyl Alcohol (Celvol 08-125) 3.43 Curesan 200 0.20Zirconia (MEI Grade MS-2) 1.38 Fused Yttria (UCM −325 mesh) 76.60Surfactant (Buntrock PS 9400) 5 drops/100 g slurry Antifoam (Buntrock AF9450) 2 drops/100 g slurry

The ingredients are added and mixed in the order shown.

EXAMPLE 10 Pre Mixed Binder (Negative Sol)

Material Weight Percent Colloidal Silica (Bindzil 830) 40.00 DistilledWater 40.00 Polyvinyl Alcohol (Celvol 08-125) 18.90 Curesan 200 1.10

These ingredients are mixed vigorously until homogeneous and then storedin a covered container at room temperature until use

EXAMPLE 11 Yttria Prime Coat Slurry (with Pre Mixed Binder from Example10)

Material Weight Percent Pre mixed binder (From Example 10) 18.18Distilled Water 3.83 Zirconia (MEI MS-2) 1.38 Fused Yttria (UCM −325)76.60 Surfactant (Buntrock PS 9400) 5 drops/100 g slurry Antifoam(Buntrock AF 9450 2 drops/100 g slurry

This slurry is similar to others in previous examples, but is made usingthe pre mixed binder of Example 10.

EXAMPLE 12 Zirconia Prime Coat Slurry (with Pre Mixed Binder fromExample 10)

Material Weight Percent Pre mixed binder (from Example 10) 14.11Distilled Water 3.53 Fused Calcium Stabilized Zirconia 82.36 (UCM CaStabilized Zirconia −325) Surfactant (Buntrock PS 9400) 5 drops/100 gslurry. Antifoam (Buntrock AF 9450 2 drops/100 g slurry

This slurry is similar to others in previous examples, but is made usingthe pre mixed binder of Example 10.

EXAMPLE 13 Zirconia +Yttria Prime Coat Slurry (with Pre Mixed Binderfrom Example 10)

Material Weight Percent Pre mixed binder (from Example 10) 15.90Distilled Water 3.89 Fused Calcium Stabilized Zirconia 40.11 (UCM CaStabilized Zirconia −325) Fused Yttria (UCM −325) 40.10 Surfactant(Buntrock PS 9400) 5 drops/100 g slurry. Antifoam (Buntrock AF 9450 2drops/100 g slurry

This slurry is similar to others in previous examples, but is made usingthe pre mixed binder of Example 10.

EXAMPLE 14 Yttria Prime Coat Slurry (Neutral Charge Sol)

Material Weight Percent Colloidal Silica (Bindzil DP-5110) 7.50Distilled Water 10.89 Polyvinyl Alcohol (Celvol 08-125) 3.43 Curesan 2000.20 Zirconia (MEI Grade MS-2) 1.40 Fused Yttria (UCM −325 mesh) 76.58Surfactant (Buntrock PS 9400) 5 drops/100 g slurry Antifoam (Buntrockaf-2004) 2 drops/100 g slurry

The ingredients are added and mixed in the order shown. This slurryutilizes a colloidal silica binder with a neutral charge. It should bestored in a closed container when not in use.

The slurries of the present invention after dipping, stuccoing anddrying are then processed in succeeding shell layers, first with one ormore intermediate coats and then any typical backup shell system. Theintermediate slurry, which is recommended for use with these and similarprimes, is formulated to minimize re-solution of the dried prime coatsduring the dipping process. The intermediate slurry contains along witha negative sol colloidal silica binder a percentage of soluble polymerfor green strength and cross linking agent (as is in the prime coat aswell) to further harden the prime coat and prevent dissolution of theorganic polymer green bond.

Two examples of intermediate alumina based slurries are shown below.

EXAMPLE 15 Intermediate Cross Linking Aqueous Slurry

Contains a cross linking, insolubilizing agent which also acts as aninorganic binder to be used with the yttria slurries of this invention.

Material Weight Percent Ammonium zirconium carbonate 18.0 Tabularalumina (−325 mesh) 82.0

This slurry will not cause any softening of the yttria prime slurry asthe ammonium zirconium carbonate (AZC) serves a dual purpose as aninorganic binder and as a cross linking insolublizer for any of theabove Examples 1, 2 and 3.

EXAMPLE 16 Intermediate Insolubilizing Aqueous Slurry

This slurry contains an organic cross-linking, insolubilizing agent toeliminate any tendencies of the Yttria slurries of Examples 1,2 and 3 tore-dissolve during the dipping process.

Material Weight Percent Colloidal Silica Sol (Bindzil 830) 16.3Distilled Water 7.0 Celvol 08-125 2.3 Insolubilizer/crosslinking agent0.2 (Curesan 200) Tabular alumina (−325 mesh) 81.8 Surfactant (BuntrockPS 9400) 5 drops/100 g slurry. Antifoam (Buntrock AF 9450) 2 drops/100 gslurry.

EXAMPLE 17 Intermediate Insolubilizing Aqueous Slurry (with PremixedBinder from Example 10)

This slurry contains an organic cross-linking, insolubilizing agent toeliminate any tendencies of the Yttria slurries of Examples 1,2 and 3 tore-dissolve during the dipping process.

Material Weiqht Percent Pre mixed binder from Example 10) 28.0 Tabularalumina (−325 mesh) 72.0 Surfactant (Buntrock PS 9400) 5 drops/100 gslurry. Antifoam (Buntrock AF 9450) 5 drops/100 g slurry.

What is claimed is:
 1. A slurry composition for forming investmentcasting molds comprising: a refractory selected from yttria, zirconia,and rare earth metal oxides; a negatively charged or neutral colloidalsilica; an organic polymer; water; and a crosslinker for the organicpolymer.
 2. The slurry of claim 1 wherein the refractory comprises fusedyttria, sintered yttria, fused zirconia, Er₂O₃, Sc₂O₃, Pr₂O₃, or La₂O₃.3. The slurry of claim 1, wherein the refractory comprises fused calciumzirconate or SrO.
 4. The slurry of claim 1 wherein said negativelycharged or neutral colloidal silica is present in an amount of betweenabout 0.5 wt. % to about 20.0 wt. % of the slurry.
 5. The slurry ofclaim 1 wherein the organic polymer is present in an amount of up toabout 3.0 wt. % on a dry weight basis.
 6. The slurry of claim 1 whereinsaid organic polymer is selected from the group consisting of latex,polyvinyl alcohol, hydroxypropyl cellulose and polymer emulsions basedon styrene, butadiene, acrylics, and vinyls.
 7. The slurry of claim 1further comprising a sintering aid in the form of a ground refractoryhaving an average particle size of less than about 2.0 microns.
 8. Theslurry of claim 7 wherein the sintering aid is present in an amount ofbetween about 0.10 wt. % to about 3.0 wt. %.
 9. The slurry of claim 7wherein the refractory material and the sintering aid are pre-mixedprior to addition into the slurry.
 10. The slurry of claim 1 whereinsaid positively charged colloidal silica, said organic polymer, saidstabilizing acid and water are pre-mixed to form a binder solution priorto addition into said slurry.
 11. The slurry of claim 1 wherein saidslurry composition has a pH of at least about 8.0.
 12. The slurry ofclaim 1 wherein said slurry composition has a pH of at least about 8.5.13. The slurry of claim 1 wherein said slurry composition has a pH of atleast about
 9. 14. The slurry of claim 1 comprising a negatively chargedcolloidal silica.
 15. The slurry of claim 1 comprising a neutralcolloidal silica.
 16. A method of preparing a casting shell for use inmolding a metal object, comprising applying at least one coat comprisinga stable slurry according to claim 1 as a prime slurry onto a waxpattern; draining excess slurry from the pattern; and stuccoing theresulting part with a refractory grain.